With the proliferation in use of wireless devices over past several years, there has been a growing interest for access to mobile Internet and web-based applications. TCP, being a popular transport layer protocol for the Internet, is primarily responsible for transfer of Internet data in heterogeneous networks comprised of wired and wireless links. However, it was originally designed to operate well in wire-line environments where the channel conditions are highly reliable and data losses are primarily due to congestion. It thus faces operational challenges in wireless scenarios that are characterized by sporadic losses and disconnections. TCP perceives the losses on a wireless link to be an indication of network congestion and invokes its congestion control mechanisms. This leads to a reduction in data transfer rate and impairment of the end-to-end throughput. The solutions that have been proposed over the past several years to counter the problem include end-to-end schemes, split connection approaches, and TCP aware link layer protocols. The end-to-end schemes as suggested, for example, in Balakrishnan et al., “A Comparison of Mechanisms for Improving TCP Performance over Wireless Links,” IEEE/ACM Transactions on Networking, 1998, and Gupta et. al., “Reliable ELN to enhance throughput of TCP over wireless links via TCP header checksum,” IEEE Global Telecommunications Conference, vol. 2, p. 1985-1989, 2002, involve making changes to TCP to make it capable of distinguishing between congestion and wireless link losses. The Explicit Loss Notification (ELN) is one such end-to-end mechanism. The ELN option is added to TCP acknowledgements. When a segment is dropped on a wireless link, the acknowledgements for the subsequent segments are marked to identify that a non-congestion loss has occurred. On receiving such an acknowledgement, the TCP source can perform retransmission of the lost segment without invoking congestion control mechanisms. Although end-to-end schemes preserve TCP semantics, these require modifications to TCP. The infeasibility of Internet wide deployment of such changes poses a severe restriction to the practical utility of such solutions. The split-connection approaches, as suggested, for example, in Bakre et. al., “Handoff and System Support for Indirect TCP/IP,” 2nd USENIX Symposium on Mobile and Location-dependent Computing, p. 11-24, 1995, and Brown et. al., “M-TCP: TCP for Mobile Cellular Networks,” ACM SIGCOMM Computer Communications Review, 1997, divide the TCP connection between the source and destination into two distinct connections—one between the source and the base station (BS) and the other between the BS and the destination. A specialized protocol tuned to the wireless environment can be used for the connection that extends over the wireless hop. The split-connection approach is, however, marred by increased processing overheads, violation of end-to-end semantics of TCP acknowledgements, and slow, complicated handoffs. Enhanced link layer reliability as suggested, for example, in Miyoshi et al., “Performance Evaluation of TCP Throughput on Wireless Cellular Networks,” IEEE Vehicular Technology Conference, 2001, has been investigated as a mechanism to improve TCP performance in wireless scenarios. However, link layer designs that are TCP unaware cannot efficiently shield TCP from the wireless losses, and are also associated with increased rate and delay variability as described in Chan et. al., “TCP/IP Performance over 3G Wireless Links with Rate and Delay Variation,” ACM MobiCom, p. 71-82, 2002. On the other hand, approaches on line of the SNOOP protocol suggested in Balakrishnan et al., “Improving TCP/IP Performance over Wireless Links,” ACM MobiCom, 1995, represent a TCP aware link layer design. While SNOOP preserves the end-to-end semantics of TCP and does not require any changes to TCP implementation, it has its own share of limitations. It cannot be used for the case when TCP data and ACKs do not both traverse through the base station BS or an access point (AP). The protocol also has overhead associated with SNOOP cache maintenance. Moreover, during the interim period between the handoffs, the Base Station (BS) or Access Point (AP) to which the handoff is occurring cannot snoop on any acknowledgements sent from the mobile host. Another disadvantage of the SNOOP protocol is its inability to function when TCP headers are encrypted.
None of the aforementioned solutions encompass or utilize adaptivity of wireless systems features like FEC, modulation transmission power, and multiple transmission modes like modulation and coding schemas. There have been some recent efforts including, for example, Laura et al., “An Analytical Study of a Trade-off between Transmission Power and FEC for TCP Optimization in Wireless Networks,” IEEE INFOCOM 2003 and Barman et al., TCP Optimization through FEC, ARQ, and Transmission Power Trade-offs, “International Conference on Wired/Wireless Internet Communications, 2004, to examine adaptive link layer measures for TCP throughput optimization. The authors adopt standard steady state TCP throughput expressions and perform optimization by adapting the coding rate, number of retransmission attempts and transmission power. The TCP dynamics and congestion control mechanisms are not considered in the work. However, for performance optimization, the link layer needs to be adaptive to the instantaneous dynamics of a TCP flow. In Singh et. al., “Channel State Awareness based Transmission Power Adaptation for Efficient TCP Dynamics in Wireless Networks,” IEEE International Conference in Communications, 2005, power control measures based on TCP's congestion avoidance dynamics have been proposed for throughput enhancement in a simplified scenario. The work does not address realistic wireless network conditions, and link adaptation based on adaptive modulation and coding is not accounted for. Furthermore the work does not have any comprehensive characterization of the congestion control dynamics of TCP.
In European Patent No. 05015951.6-, a method and system is proposed to model TCP throughput and to evaluate power control measures to compensate for fading and path loss for highly mobile systems. The work does not model the dynamics of TCP and uses a steady state TCP throughput expression to evaluate power adaptation policies to enhance TCP throughput. Link adaptation measures like adaptive modulation and coding are not explored in the work.